All industrial processing of ground cereal products with the addition of water to a product water content of more than 25% is confronted by a number of problems. The first is the question of which structure the product is to have after processing. This concerns the protein in particular. Further, a very important factor in processing is the state of the starch, whether it is to be raw or cooked or gelatinized after processing. Another question concerns the outer form of the final product. In particular, specific requirements arise from the biological and biochemical peculiarities of the ground cereal products, e.g. in relation to cleaning and maintaining the cleanliness of the plant. Hygiene is very important in this respect. All microbial spoilage must be avoided. The risk of such spoilage is high because of the high product moisture and a temperature (20.degree.-40.degree. C.) of the freshly moistened product which is "ideal" for the propagation of harmful microbes.
As long as the product to be ground is still in its natural state, the protein forms biochemical bonds with the added water and mechanical (kneading) action. A protein structure which gives the final product a good stability of shape, cooking stability and good texture when chewed, for instance, develops again between the flour particles which have been crushed by grinding.
The problems particular to the production of a product in the form of small lumps will now be discussed in the following with reference to the fabrication of couscous. Couscous is an industrially produced pre-cooked cereal product which, in its ready-to-eat state, is of a quality very similar to rice. In contrast to the grinding product (flour, middlings, semolina) as such, the couscous can be conserved for a long time and stored like pasta when the product moisture is less than 12%.
An industrial process for couscous production is described in the Swiss Patent CH-PS 612 835. The raw material (flour, semolina or middlings) is mixed well in exact proportions with a corresponding addition of water to a mixing trough with a slowly revolving mixer shaft for approximately 14 to 15 minutes. In so doing, lumps of approximately 10 to 40 mm in size are formed and subsequently reduced in a centrifugal beating device to a size of less than 6 mm. Endeavors were made to steam or gelatinize the individual parts in a size equal to or greater than the corresponding dimensions of the finished product particle.
The desired size of the granulate is achieved only after the drying process by reducing the agglomerate. The unwanted fine portion is fed to the raw material. In practice the best final product quality can be achieved in this way. However, returning 14 to 20% of the fine portion was found to be a substantial disadvantage, particularly since the output capacity of the entire plant is reduced by this percentage. In order to overcome this defect the product was consequently sifted while wet prior to steaming and only the fraction with an agglomerate size of 1-4 mm was steamed. All oversized agglomerates are returned to the mixing trough. In this way, also, a final product of adequate quality could be obtained and the ratio of batches for both steaming and drying could be optimized. However, two new obstacles were encountered. The quantity of oversized agglomerates which had to be returned to the mixing trough was so large that the output of the latter had to be doubled. The wet sifting caused problems in that the sieve had to be cleaned and even exchanged at frequent intervals to prevent a stopping of the sieve.